Radio transmitter



Feb. 12, 1935. v R, E I 1,990,585

RADIO TRANSMITTER Inventor: Robert? B. Don'we,

b (Al/66w v H i s Attovneg.

Feb. 12, 1935. B, DQME 1,990,586

'RADIO TRANSMITTER Filed April 28, 1932 2 Sheets-Sheet 2 f= ZeroTnvntort Robert B- Dome JQQMWOM H i s Aftorn ey- Patented Feb. 12, 1935UNlTED STATES RADIO TRANSMITTER Robert B. Dome, Schenectady, N. Y.,assignor to General Electric Company, a corporation of New YorkApplication April 2c, 1932, Serial No. 608,024

3 Claims.

My invention relates to radio transmitters and more particularly totransmitters of the high power shortwave type.

It has for one of its objects to provide means whereby currents, whichas originally produced are extremely feeble and include frequenciesextending over a very wide range, as for example, from frequenciesrelatively low in the audio range to frequencies relatively high in theradio range, may be faithfully transmitted by a high power short wavetransmitter. Such currents are produced, for example, by thephoto-electric cells employed in television apparatus. The currentsflowing in these cells may include frequencies extending from about 20cycles to a million or two million cycles. It is of course desirableto'transmit the widest possible range of these frequencies to thereproducing equipment. Difficult problems, however, are encountered incausing a radio transmitter, particularly one operating at ultra shortwave lengths, and of high power rating, so to transmit currents having arange of frequencies greater than from 20 to 50,000 or 75,000 cyclesthat faithful reproduction of all of the signal currents in thefrequency band is possible at a remote point.

Some realization of the problem encountered may be had from aconsideration of the fact that a variation in frequency from 20 to1,000,000

cycles, for example, amounts to a variation in frequency of 5,000,000percent. To construct an amplifier to operate at frequencies which varyover such a range and to amplify faithfully to a high power level theentire range of frequencies has in the past been impracticable.Amplification of these currents at this frequency and varying over thisfrequency range to a high power level may be avoided in a great portionof the frequency range by modulating the short wave transmitter at a lowpower level and thereafter amplifying the modulation products to a highpower level. At ultra high frequencies, as for example, at wave lengthsless than meters, this does not avoid the difficulty arising in theconstruction of the amplifiers, for the reason that it is alsoimpracticable at ultra short wave lengths to construct amplifiers whichfaithfully amplify the range of frequencies included in the modulationproducts.

-One of the objects of my invention is to provide a method and meanswhereby these difficulties are overcome.

In accordance with a further object of my invention a method and meansare provided whereby the signal currents are converted'at a l w powerlevel to an intermediate portion of the frequency spectrum where theentire frequency range may be faithfully and'practically amplified tothe desired high power level. They are then amplified to the desiredpower level and reconverted at the high power level to the originalportion of the frequency spectrum. The ultimate carrier wave comprising,of course, but a single frequency, may also be amplified to the desiredhigh power level. At this high power level the carrier wave may bemodulated with the high power reconverted signal currents and radiatedfrom the antenna.

A further object of my invention is to provide means whereby the lowpower signal currents may be faithfully amplified to a level at whichsatisfactory conversion at low power to the intermediate portion of thefrequency spectrum may be effected and whereby after reconversion theultimate carrier wave maybe faithfully modu= lated thereby.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My inventionitself, however, both as to its organization and method of operation,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawings in which Figs. 1 and.2 representdifferent embodiments of my-invention; Fig. 3 represents an equivalentcircuit of a portion thereof; and Figs. 4 and 5 represent certaincharacteristics with reference to its operation.

Referring to the drawings, I have shown in 1 including a source of shortwave high power radio frequency oscillations conventionally indicated at2. This source may of course comprise a conventional short wavegenerator and series of amplifiers whereby the generated oscillationsare raised to a high power level. The output from this source issupplied to the grids of a pair of push pull connected electrondischarge devices 3 and 4 by which the high frequency oscillations areamplifiedand supplied to Fig. 1 thereof a conventional radio transmittera radiating system 5. While I contemplate that to be televised with aspot of light. Currents are then produced in the photo-electric cells 6maccordance with light falling thereon. These currents may includefrequencies extending over the above-mentioned range of frequencies.They are extremely feeble as is of course well known in the artpertaining to photo-electric devices. It is necessary that they beamplified to such an extent that satisfactory modulation of the radiotransmitter 1 in accordance with these currents may be effected. It isalso necessarythat this amplification shall be effected in such a waythat all of the frequencies within the desired range are amplified to anequal extent and phase displacement due to the operation of theamplifying means is wholly avoided. Unless the amplifying means be suchthat these results are effected then the transmitter is not modulatedfaithfully in accordance with the currents generated by thephoto-electric cells and the picture which is produced by any remotereproducing equipment will not be a faithful reproduction of the objecttelevised.

In accordance with my invention the signal currents are converted to aportion of the frequency spectrum in which the highest frequency to beamplified is but a very small percent greater than the lowest frequencyto be amplified but where linear amplification of the entire range ispracticable. After this conversion the currents in this relativelynarrow range of frequencies may be satisfactorily amplified to thedesired high power level by ordinary amplifiers.

For this purpose I provide a source of oscillations of a frequencywithin the mentioned intermediate portion of the frequency spectrum,such for example as 20,000 kilocycles. This source may comprise aconventional crystal controlled elec-. tron discharge oscillator 13, theoutput of which is amplified by amplifiers 14 and 15, in the output ofthe latter of which these oscillations are modulated by oscillationssupplied from the photo-electric cells through amplifiers 10, 11, and12. If we assume now that the currents generated by the photo-electriccells vary over a range of from 20 to 1,000,000 cycles then the lowestfrequency in the lower side band of the modulation products appearing inthe output of the amplifier 15 is 19,000 kilocycles, whereas the highestfrequency produced in the upper side band is 21,000 kilocycles. Theseferquencies vary from the carrier frequency by only five percent.Eventhis narrow range may be reduced on a percentage basis by employinga higher intermediate frequency carrier wave. Thus the desired range offrequencies when converted to this portion of the frequency spectrum maybe very readily and faithfully amplified by amplifiers of ordinaryconstruction. The modulated oscillations appearing in the output of theamplifier 15 are amplified by amplifiers 16 and 17 of conventionalconstruction. The amplifier 17 is represented by a rec tangle and mayinclude as many stages of amplification similar, for example, withamplifiers 15 and 16, as are necessary to amplify thedesired currents tothe required power level. I

Prior to modulation of the transmitter 1, how

ever, it is necessary that these currents be reconverted to the originalportion of the frequency spectrum. This is accomplished by electrondischarge devices 18 and 19 the grids of which are connected in pushpull relation to receive oscillations from the output of the amplifier17. The anodes of these devices are connected in parallel through acircuit which includes a radio frequency choke coil 19, an iron coremodulation reactor 20, and a source ofelectromotive force 21. The gridsof these discharge devices are connected to the cathodes throughopposite portions of an inductance 22 and a source of grid biasingpotential 23. The discharge devices 18 and 19 are of course of highpower rating comprising tubes of the type, which are commonlyconstructed with water cooled anodes. The source of potential 23,however, is such that the discharge devices are biased strongly negativethereby to cause them to act as giant detectors to demodulate theoscillations supplied to the grids. Thus in the output circuits of thesedischarge devices appear oscillations having the same frequency as thoseproduced by the photo-electric cells, these oscillations having beenamplified many fold. The anode circuit of the power amplifiers 3 and 4of the radio transmitter are then modulated by the products of thisdemodulation by means of a circuit extending from the anodes of devices18 and 19 to the anodes of devices 3 and 4 and including an inductance24, a parallel combination of a voltage reducing resistance 25 and abypass capacitor 26 and a. conductor 28 extending to the midpoint of theanode inductance 29 of the push pull power amplifier. In this way thelow frequency electromotive forces developed across the modulationreactor 20 are supplied directly to the anodes of the push pullamplifier thereby varying the amplification of these amplifiers inaccordance with the signal currents. The modulated radio frequencymillations are then supplied to the antenna 5.

It will be noted that the currents generated in the photo-electric cellsare amplified to a certain extent prior to modulation of theintermediate frequency carrier wave generated by the oscillator 13. Thisis necessary to practical modulation of the intermediate carrier waveeven at a low power level. A special construction of the amplifiiershowever, is necessary in order that the currents will be amplifiedequally over the entire range of frequencies and to avoid phasedisplacements which are likely to result. For this reason I have shownin detail the circuit construction between the anode of the dischargedevice 11 and the grid of the discharge device 12. It will of course beunderstood that any additional amplifiers represented by the rectangle10 are of similar construction.

The anode circuit of the device 11 is connected to the grid of thedevice 12 through an air core inductance 30, a coupling capacitor 31,and a parallel combination-including resistance 32 and an iron coreinductance 33. The condenser 31 is the conventional coupling capacitorwhich serves to isolate the grid of the discharge device 12 from thehigh unidirectional electromotive force which is'supplied to the plateof the discharge device 11 by the source 34. This condenser, forexample, may be of about 4 microfarads.

In the higher portion of the frequency range which is to be amplified itis found that the capacity effects between the grid and cathode of thedischarge device 12 and stray effects between the conductors and in thecircuit connections generally, greatly impair the transmission ofeffectively to cut off at a frequency well within the range where a highdegree of amplification is desired. It has been found, however, thatthis cutting offat the higher frequencies can be effectively avoided bythe use of the inductance 30 having a value such that it resonates withthe effective total capacitance between the grid and the cathode of thedischarge device.

The effect of this inductance may best be understood by reference toFigs. 3 and 4. Fig. 3 represents the equivalent circuit included betweenthe discharge devices 11 and 12. This equivalent circuit includes asource of electromotive force 34 which, of course, corresponds to theelectromotive force generated in the anode circuit of the dischargedevice 11. This source of electromotive force may be considered asconnected in a circuit including a resistance Rp,

which is the internal static resistance of the plate circuit of thedischarge device 11, and a parallel combination of resistance R1; andcapacitance C. The resistance R includes the resistance between the gridand the cathode of the discharge device 12 and comprises the resistance35. The capacitance C is the capacitance between the grid and thecathode and that which is included in the circuit connections to theseelectrodes. This equivalent circuit, however, ignores the elements 31,32, 33, and 40, except in so far as their stray capacity to groundeffects the value C. With this exception these elements have little orno effect in the high frequency range. The impedance Z1 looking into thegrid of the discharge device 12 as indicated in Fig.

3 may be expressed as follows: Z1=R1-iwx1.f

When R1 is the resistive component of the total input impedance, X1 isthe capacitive component;

a=21|r times the frequency and The relation between the variables 21,.R1, and

X1 at the different frequencies may best be understood from aconsideration of Fig. 4 in which this relation is shown vectorially. Itwill of course be understood that at zero frequency the capacitance hasno effect and the impedance Z1 is purefresistance and may be represented'by the vector R0; equal to the resistance of the pathincluding'resistor 35. As the frequency increases theresistance 35becomes less efiective and the capacitance qbecomes increasinglyeffective with the resultthat the resistive component R1 decreases andthe reactive componentX1 increases. Thus, for example, at a certain lowfrequency the impedance Z1 may be represented by the vector A, thisimpedance being made up of a resistance component R and a reactancecomponent X1 indicated in the diagram respectively as the horizontal andvertical sides of the triangle of which the vector A is the hypotenuse.At a still higher frequency this impedance Z1 may be. represented by thevector B comprising a smaller horizontal component R1 and a largervertical component X1. At infinite frequency it will of course beunderstood that the impedance of the capacitance is zero and accordinglythe resistance R0 is of no effect and the impedance Z1 is zero. It will.thus be seen that the locus of the various vectors Z1 drawn atdifferent frequencies comprises for practical considerations, asemicircle D. The maximum reactive component of the impedance Z1 occursat the point where the resistive and reactive components are equal.

Thus as the frequency increases to-a point where R1 equals Rn z theimpedance Z1 becomes gradually more reactive until a maximum reactivecomponent is reached.

The problem then is to insert an additional 5 corrective element in thecircuit which so effects the total reactive component of the impedanceinto which the tube 11 works as to produce on the grid of tube 12 avoltage, which, at the highest frequencies to be amplified is equal tothat produced at the frequencies low in the audio range. Since thereactive component of the impedance Z1 is capacitive the correctiveelement should be inductive and is indicated in the drawings at 30. 15

In setting up the circuit the only two known quantities are the internalresistance Rp of tube 11 and the input capacity to tube 12. The valuesof resistance 35 and inductance 30 are to be determined with referenceto these quantities.

The value of resistance 35, or R0, may readily be determined as follows:

The impedance Z1 is of course The resistance component of-thisexpression of Z1 is and the reactance component of Z1 is "(UCRQ 1+w C RAt the critical frequency, 1. e. the highest frequency to be amplifiedR1=X1 as shown by Fig. 4. Therefore 1+w CR l+w C R Solving for R0 wefind To determine the value of inductance of the corrective element 30let us assume that at the critical frequency it has an unknown reactanceX1. as indicated in Fig. 4.

The total impedance Z11 in which the tube 11 works is then Thequantities .X and Z11 are also shown in Fig. 4.

The voltage E32 on the grid of tube 12 is the voltage E32 is For uniformamplification the -absolute value of Eu at the highest frequency to beamplified and at low frequencies should be equal.

That is,

This is a double valued function. But from the above relation since itgives a value Xr=0 at low frequencies.

NOW

Ro1/R where f is the highest frequency to be amplified. This equationexpressed the general value of L for uniform amplification.

In a particular case we may arbitrarily make XL=X1 thereby to give tube11 a unity power factor load. With this value XL to obtain uniformamplification the value of R0 as found in Equation 5 must be adjusted inaccordance with Equation 16. That is to make X equal to zero R0 mustequal 2R or Thus we have the following equations from which the circuitmay be designed for uniform amplification, with unity power factor loadon tube 11 R=JER, (21) Where a unity power factor load on tube 11 is notimportant, to obtain uniform amplification the circuit may set up inaccordance with the following equations Piano-W (23) When Xx. equals X1the added inductance 30 resonates with the input capacity to tube 12. Inpractice it has been found that with the inductance so chosenpractically uniform amplifica tion may be obtained over the entire rangeextending from very low audio frequencies to relatively high radiofrequencies as for example frequencies of the order of 1,000,000 cycles.Further, this value gives the maximum amplification over this range.This is apparent from Fig. 4 since when X1. equals Xi, Z11 is a minimumand identical to R1 and the ratio of Z1 to Zn is maximum therebyproducing a maximum voltage on the grid of tube 12.

The reactance Xi. for maximum amplification at the different frequenciesis shown by the curve E which is so drawn as to indicate that XL at thecritical frequency equals X1. That is, the inductance 30 is in resonancewith the effective input capacity to tube 12. At lower frequencies itwill be observed that the inductive reactance X1. approximately balancesthe capacitive reactance X1. It has been found in practice that the sumof XL and X1 so closely approximates zero at frequencies lower than thecritical frequency that the reactive component of the impedance Z1inthis range of frequencies has substantially no effect upon gridvoltage supplied to the tube 12, with the result that for practicalactance at the highest frequency to be amplified the amplifier operateswith substantially equal amplification over the entire range and cutsof! rapidly at frequencies above that range.

. It has been found that with R0 greater than at the critical frequencyunder unity power factor conditions the voltage supplied to the grid ofdevice 12 increases at the high frequencies thereby increasing theamplification of the system. Similarly if R0 at the critical frequencybe less than under unity power factor conditions then the voltagesupplied to the grid of the device 12 is reduced at the highfrequencies. While a uniform amplification characteristic is usuallydesirable, in some cases it is desirable to produce either a rising orfalling amplification characteristic at the high frequencies thereby tocompensate for an undesired amplification characteristic obtained insome other part of the equipment.

I 1,980,686 This may be effected by proper choice of R1.

Of course, the same difficulty is encountered in connectionwith-modulation of the power amplifier 3, 4 and for this reason theinductance 24 is employed. This inductance is constructed to exactlyneutralize the capacitive component of the impedance which appearsbetween the midpoint of coil 29 and the cathodes of devices 3 and 4. Inthis case the resistance R0 becomes the value where Eb is the averageanode voltage supplied to the power amplifier and 1b is the averageanode current.

At very low frequenciesthe capacitance of the coupling capacitor 31between discharge devices 11 and 12 tends to shift the phase of theoscillations supplied to the grid of the discharge device 11; Thisshifting of the phase is of course intolerable since it seriouslyimpairs the picture which is reproducedby the remote receivingapparatus. I For example. if a television receiver of the cathode raytype be employed the flying spot is either advanced or retarded from itstrue instantaneous position due to this phase displacement therebygreatly impairing the reproduced image. To avoid this effect theinductance33 shunted by a resistance 32 is included in the circuit, theinductive reactance of this combination having such a value that itresonates with the capacitance of condenser 31 at a frequencyapproximating the lowest frequency to be amplified. The purpose ofresistance 32 is to prevent the inductance 33 from having an appreciableeffect upon the amplification of the system at the higher frequencies.

The effect of this combination is best, illus trated in Fig. 5 where Ihave shown the curve F showing the relation betweenthe capacitivereactance of the condenser 31 plotted as ordinates and frequency plottedas abscissa. It will be seen that at zero frequency the reactance isinfinite and that it rapidly decreases in accordance with the curve asthe frequency increases. By the curve G I have shown the relationbetween the inductive reactance of the combination 32, 33 and frequency.It will be seen that at zero frequency this reactance is zero and thatit rapidly rises to a certain value where the resistance 32 becomeseffective to shunt the reactance. At the higher frequencies thisresistance becomes increasingly effective with the result that theinductive reactance reaches a maximum value and then drops off inaccordance with the curve G. The total reactance of the condenser 31 andthe combination 32, 33 is thus the sum of the reactances represented bythe curves G and F and is shown by the curve H. It will be observed thatat a certain frequency ii the inductive reactance is equal to thecapacitive reactance, At the higher frequencies there is a slightinductive reactance. It has been found, however, that this reactance maybe made so small over the range where the capacitance of condenser 31 iseffective as to pre vent any appreciable shift in phase of theoscillations supplied to the grid of device 12.

To obtain even better operation the combination 32, 33 may be shuntedbya condenser 40 this condenser having such a value that it issubstantially ineffective except at very high frequencies. Thiscondenser then acts as an additional shunt to the inductance 33 therebyfurther tending to reduce the impedance of the combination 32, 33,

40 to zero at the highest frequency to be amplified. In this wayincreased amplification is obtained particularly in the extremely highportion of the frequency range.

If it is desired to amplify current of from 20 cycles up, for example,the frequency f1 may be of about 24 or 25 cycles. Then, at frequenciesbelow about 20 cycles the total. reactance of the circuit increasesextremely rapidly thereby ren-- dering the amplifier inoperative at theextremely low frequencies.

It will be readily seen from Fig. 4 that with the circuit constants ofthe amplifiers 11 and 12 chosen as described the system is operative toproduce uniform amplification over a practical maximum range offrequencies. If the inductance 30 has a value such that it resonateswith the effective input capacity of device 12 at a frequency greaterthan the frequency at which uniform amplification will not be had overthe entire range of lower frequencies. Instead the system would operatemore nearly in the nature ofa tuned amplifier having selectivity atparticular high frequencies.

In Fig. 2 I have shown a system in accordance with my invention adaptedfor reception and retransmission of signals of the type herein referredto. Such systems are necessary particularly when short wave lengths ofthe order of 5 meters are employed since waves of extremely short lengthbehave in the nature of light waves and rebroadcasting is necessarywhere such waves cannot travel in a straight line between thetransmitting and receiving points.

The receiving equipment comprises an antenna 36 whereby thereceivedoscillations are supplied to the grid of a detector 37. Heterodyneoscillations are also supplied to the grid of the detector 37 from aheterodyne source 38. In the output circuit of the detector 37 afrequency is produced which may, for example, be of about 20,000kilocycles or any other suitable frequency where amplification may bereadily effected. The remainder of the system is then the same as hasbeen described in connection with Fig. 1 and comprises amplifiers 16 and17, detectors 18 and 19, and the conventional radio transmitter 1.

While I have shown particular embodiments of my invention it will ofcourse be understood that I do not wish to be limited thereto since manymodifications may be made both in the circuit arrangement and in theinstrumentalities employed. I contemplate by the appended claims tocover any such modifications as fall within the true spirit and scope ofmy invention. I

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In combination, a source of oscillations having a wide ;range offrequencies including frequencies in the audio range and frequenciesrelatively high in the radio range, an electron discharge device,operating connections therefor including connections whereby said highfrequency oscillations are supplied to the electrodes quencyoscillations are supplied to said discharge device said inductancehaving such a value relative to the resistances of the input circuit tosaid discharge device that all frequencies within said range aresupplied to said lectrodes with substantially equal transmission eilciency. I

2. In combination, a source of oscillations having a wide range offrequencies including frequencies in the audio range and Frequenciesrelatively high in the radio range, an electron discharge device,operating connections therefor including connections whereby said highfrequency oscillations are supplied to the electrodes of said dischargedevice, said range of frequencies being such that in the higher portionof said frequency range capacity effects between electrodes of saiddischarge device and in said operating connections reduce the efliciencyof transmission of said oscillations to said electrodes, and aninductance included in said connections whereby said high frequencyoscillations are supplied to said discharge device, said inductancehavin such a value that it resonates with said capacity effects atsubstantially the highest frequency in said range, and the inputresistance to said discharge device being adjusted to equal /2 times theresistance of the source of said oscillations.

3. In combination, a carrier wave amplifier, comprising an electrondischarge device having an anode, a cathode, and a grid, operatingconnections therefor including means to supply a carrier wave to beamplified to said grid, a source of signal oscillations, and connectionsfrom said source to said cathode and anode whereby said carrier wave ismodulated with said signal oscillations, said signal oscillationsincluding frequencies extending over a wide range such that capacityeffects between the electrodes of said discharge device and in saidoperating connections prevent efficient transmission of signaloscillations in the high frequency portion of said range to said cathodeand anode, and an inductance included in said connections from saidsource having such a value relative to the resistances and capacity ofthe circuit that all frequencies within said range are supplied to saidcathode and anode with substantially equal transmission efficiency.

4. In combination, a pair of electron discharge devices connected incascade for successive amplification of signal currents having a widerange of frequencies, an inductance connected between the anode of oneof said devices and the grid of the. other device having such a valuethat it resonates with the effective capacity of the input circuit tosaid other discharge device at the highest frequency to be amplified,and a resistance connected between the grid and cathode of said seconddevice having a value at said highest frequency equal in magnitude tothe input circuit capacity reactance of said second tube.

5. In combination, a pair of electron discharge devices, each of saiddevices having an anode, a cathode, and a grid, a source of oscillationsconnected between the grid and cathode of one of said discharge devices,a resistance connected between the grid and cathode of the other of saiddischarge devices, and an inductance connected between the anode of saidone of said devices and the grid of the other device having such valuethat it resonates with the effective capacity between the grid andcathode of said other device at the frequency where the effectiveresistance between the grid and cathode of said other device is equal toone half of the value of said resistance connected between the grid andcathode at zero frequency.

6. In combination, an electron discharge device having a cathode and agrid, 9. source of oscillations having desired frequencies extendingover a wide range, connections between said source and said cathode andgrid including a coupling capacitor, and a pair of inductive impedancesall connected in series, one of said impedances having a value such thatit resonates with the capacity of said coupling capacitor at a frequencylow in said range and the other of said impedances having such a valuethat it resonates with the effective capacity between said grid andcathode at a frequency high in said range.

7. In combination, an electron discharge device, having a grid circuitand an anode circuit, said grid circuit including av source ofoscillations to be repeated to said anode circuit and including a widerange of frequencies, and a coupling capacitor having one electrodeconnected to said source and another electrode connected to said gridwhereby said source is coupled to the grid of said discharge device, andan inductive impedance in said circuit resonant with the capacitance ofsaid coupling capacitor at a frequency low in said range said impedancebeing connected in series between said source and said grid.

8. In combination, an electron discharge device, having a grid circuitand an anode circuit, said grid circuit including a source ofoscillations to be repeated to said anode circuit and including a widerange of frequencies a coupling capacitor whereby said source is coupledto the grid of said discharge device, an inductance connected in serieswith said source and said grid to neutralize the reactance of saidcoupling capacitor at low frequencies, and means substantially toprevent said inductance from having an appreciable eilect in the highfrequency portion of said range.

9. In combination, a pair of electron discharge devices, each of saiddischarge devices having an anode circuit and a grid circuit, a sourceof oscillations having frequencies both low in the audio range andrelatively high in the radio range connected in the grid circuit of oneof said devices, a connection between the anode circuit of said one ofsaid devices and the grid circuit of the other device including acoupling capacitor, an iron core inductance and an air core inductanceall in series, said inductances having such values relative to thecapacitances of the circuit and connections including the grid of saidsecond discharge device that said oscillations are repeated to the anodecircuit of said second discharge device with substantially equalfidelity throughout said frequency range.

ROBERT B. DOME.

Certificate of Correction Patent No. 1,990,586. February 12, 1935. I

v ROBERT B. DOME It is herebyicertified that errors appear in theprinted specification of the above numbered patent requiring correctionas follows: Page 3, second column, lines 70 to 74, strike out theformula and Insert mstead ll o and page 6,' second column,vljne 41,claim 8, after frequencies insert a comma; and that the said LettersPatent should be read with these corrections therein that the same mayconform to the record of the case in the Patent Ofiice.

Signed and sealed this 19th day of March, A. D. 1935.

[SEAL] I LESLIE FRAZER,

Acting Commissioner of Patents.

